The present invention relates to a self-healing ring network and a method for fault detection and rectifying in said network.
Wave Division Multiplexing (WDM) is a technology allowing the transmission of a number of optical signals through an optical fibre using different separated light wavelengths. In this way the information carrying capacity may be increased significantly. The capacity depends on the number of used wavelength channels and their bandwidth. The signal at each wavelength travels through the fibre irrespectively of the other signals, so that each signal represents a discrete channel with large bandwidth.
A ring communication network is made up of nodes that are connected in tandem in a ring by a unidirectional communication path, such as an optical fibre. A node receives transmissions from an upstream node. The return traffic is transmitted downstream to the first node
A drawback of such a network is that a break in the ring or a failure of a node would prevent any node upstream of the break/failure of communicating with any node downstream of the break. A usual solution to this problem is to in some way providing a second spare communication path parallel to the first, but in the opposite direction, see U.S. Pat. No. 5,365,510, U.S. Pat. No. 5,179,548 and EP 677,936. If communication fails somewhere on the first communication path, then the traffic is directed back on the second communication path and the wished node will thus be reached from the other side.
Another solution is to send traffic on two communication paths in opposing directions, but with a segment of the ring inactivated for data traffic, see xe2x80x9cElectronic lettersxe2x80x9d, Dec. 5th 1996, Vol.32, No 25, p 2338 -2339, B. S. Johansson, C. R. Batchellor and L. Egnell: xe2x80x9cFlexible bus: A self-restoring optical ADM ring architecturexe2x80x9d. In the case of a fault the segment is moved to the fault. It is however not described how to achieve this in practise.
In U.S. Pat. No. 5,113,459 is shown an idea of using a special channel for optical service signalling.
The purpose with the present invention is to solve the problem of a practical implementation of the self-healing ring network described in xe2x80x9cElectronic lettersxe2x80x9d, Dec. 5th 1996, Vol.32, No 25, p 2338 -2339, B. S. Johansson, C. R. Batchellor and L. Egnell: xe2x80x9cFlexible bus: A self-restoring optical ADM ring architecturexe2x80x9d, which hereby is incorporated by reference. A ring network will be called xe2x80x9cringxe2x80x9d for short.
The present invention transmits in WDM channels on two communication paths in opposing directions at the same time, but with an inactive segment on the ring, which prevents nodes on the ring from receiving traffic from more than one direction at the time. In the case of a fault, the inactive segment is moved to the fault.
The problem of accomplishing an inactive segment may be solved by using switches in the nodes, but a more economic and reliable way is to use amplifiers in the node, which are needed anyway, as switches. The switching means block transmission towards or reception from the segment of the ring that is an inactive segment. An advantage of blocking reception instead of transmission is that signals is then present at the input of the node. The signals may then for example be used for controlling the gain of a switched off amplifier, so that the amplifier has the proper gain when it is switched on again. It will also make it easier to discover a fault in the inactive segment.
The problem of how to signal when and how to move the inactive segment is solved via a supervisory channel on a wavelength separated from the data traffic channels. The supervisory channel never really enters the node, but is received and retransmitted outside the main parts of the node. The advantages with that is that the supervisory channel does not take power in the nodes and it does not disturb the other channels. It is also possible for the supervisory channel to pass the inactive segment, which may be essential in the event of a fault and for line amplifier management.